The present invention is related to exposing imageable material using multiple exposing beams, and in particular to processing data for computer-to-plate (CTP, C2P) ablative imaging with multiple energy beams, e.g., multiple laser beams to prevent visible stitching lines appearing between the individual multiple beam packets.
One application of ablative imaging is flexography, one of the main known printing processes. A flexographic (“flexo”) plate or plate/sleeve combination, normally rubber or photopolymer plate, is fabricated in such a way that the areas corresponding to zones to be inked are geometrically higher than the areas corresponding to zones not to be inked. Contacting the flexographic plate or plate/sleeve combination with an inking roller, such as an anilox roller, inks the flexographic plate. Only the geometrically higher zones of the flexographic plate or plate/sleeve combination are inked, other areas are not inked. Subsequently, the inked flexographic plate or plate/sleeve combination is brought in contact with a substrate and the inked parts transfer ink onto the substrate, thus producing the desired image on the substrate.
Ablative media includes plates and plate/sleeve combinations that are designed for direct computer-to-plate exposing of plates according to imaging data provided in digital form. For example, ablative flexographic plates designed for CTP imaging are typically photopolymer plates that are pre-sensitized with a Laser Ablation Mask System (LAMS) coating.
There is efficiency to be gained by imaging using multiple beams. Thus, a plate is exposed by multiple beams, e.g., modulated laser energy beams that simultaneously form packets of tracks on the plate. Relative motion is produced between the plate and the multiple beams in both a fast scan direction in which several tracks are laid simultaneously, and a slow scan direction substantially perpendicular to the fast scan direction. FIG. 1 shows a typical imaging exposure in which a number of beams, 4 in FIG. 1, are laid simultaneously in the fast-scan direction. This forms a packet of tracks of width four times the slow-scan-direction track separation.
Transferring imaging data onto a media by means of ablative imaging with multiple beam methods often results in a visible stitching line in the areas between the individual multiple beam packets. Therefore, stitching artefacts may appear in these areas, called stitching areas, near the borders of the packets of tracks, as shown in FIG. 1.
The stitching lines result in a combination of inaccurate imaging and re-condensation of ablated material on the already ablated multiple beam tracks. These stitching lines disturb the homogeneity of the imaging appearance to the human eye.
In the case of digital flexographic imaging, the stitching lines prevent the underlying photopolymer from being completely polymerised during main exposure under UV light, thus leaving fine grooves on the surface of a completely processed digital flexographic plate. Such grooves influencing the ink transfer for some substrate-ink combinations, thus are often regarded as plate defects.
Furthermore, it is thought that a regular set of grooves influences the ink transfer depending on the orientation of the grooves towards the printing direction. Typically, an orientation parallel to the printing direction results in more homogeneous ink transfer than an orientation perpendicular to the printing direction. As flexo plates are expensive, the orientation of the imaging data—and thus the orientation of the grooves—is often changed at the CTP system to reduce plate wastage. It is therefore desirable to avoid grooves being introduced by the CTP imager itself, in order to achieve consistent printing quality.
Thus there is a need in the art for a method and apparatus of exposing using multiple laser energy beams that prevents such stitching lines from becoming detectable by the human eye in the resulting plates as well as in prints made from the resulting plates.
This problem has been recognized before. U.S. Pat. No. 5,818,498 to Richardson, et al. describes one method of avoiding stitching by imaging the stitching area at least twice by using more imaging beams than the actual advance in slow scan direction. FIG. 2 shows an example of imaging using the Richardson, et al approach with 4 multiple beams with a single overlap. Such Richardson, et al approach has the disadvantage that more beams are used than necessary for the imaging advance, thus the laser power has to be distributed to more channels, leading to laser power loss in each channel. If n denotes the number of tracks in the slow scan direction in each packet, e.g., 4 in FIG. 2, and m denoted the number of double imaged tracks per packet, e.g., 1 in FIG. 2, the maximum productivity of the imaging system is reduced by n/n, e.g., by ¼ in FIG. 2. This is significant when imaging using laser-power limited imaging systems such as is often the case in imaging digital flexographic plates. Furthermore, double-imaging a track with the same data on an ablative media may, and typically does result in a different appearance of such a track compared to those tracks that were imaged only once.
It is desirable to so prevent the stitching being visible without increasing the laser power used in exposure.